Tutorials and Topics - Peabody Computer Music

Tutorial 9Synthesis:Amplitude modulationAmplitude modulation produces sidebands—additional frequencies not present in thecarrier or the modulator—equal to the sum and the difference of the frequencies presentin the carrier and modulator. The presence of a DC offset (technically energy at 0 Hz) inthe modulator means that the carrier tone remains present in the output, too (which isnot the case with ring modulation).• Click on the message boxes containing the numbers 32, 50, 100, and 150, in turn. Youwill hear the carrier frequency, the modulator frequency (which is now in the low endof the audio range), and the sum and difference frequencies.When there is a harmonic relationship between the carrier and the modulator, thefrequencies produced belong to the harmonic series of a common fundamental, and tendto fuse more as a single complex tone. For example, with a carrier frequency of 1000 Hzand a modulator at 250 Hz, you will hear the frequencies 250 Hz, 750 Hz, 1000 Hz, and1250 Hz— the 1st, 3rd, 4th, and 5th harmonics of the fundamental at 250 Hz.• Click on the message boxes containing the numbers 200, 250, and 500 in turn to hearharmonic complex tones. Drag on the “Tremolo Depth” number box to change thedepth value between 0. and 0.5, and listen to the effect on the relative strength of thesidebands.• Explore different possibilities by changing the values in the number box objects. Whenyou have finished, click on the ezdac~ to turn audio off.It is worth noting that any audio signals can be used as the carrier and modulator tones,and in fact many interesting results can be obtained by amplitude modulation withcomplex tones. (Tutorial 23 allows you to perform amplitude modulation on the soundcoming into the computer.)SummaryThe amplitude of an audio (carrier) signal can be modulated by another (modulator)signal, either by simple multiplication (ring modulation) or by adding a time-varyingmodulating signal to a constant signal (DC offset) before multiplying it with the carriersignal (amplitude modulation). The intensity of the amplitude modulation can becontrolled by increasing or reducing the amplitude of the time-varying modulator relativeto its DC offset. When the modulator has a DC offset, the carrier frequency will remainpresent in the output sound, along with sidebands at frequencies determined by the sumand the difference of the carrier and the modulator. At sub-audio modulatingfrequencies, amplitude modulation is heard as tremolo; at audio frequencies the carrier,modulator, and sidebands are all heard as a chord or as a complex tone.111